OAuth 2.0 Device Authorization Grant

Abstract

The OAuth 2.0 device authorization grant is designed for Internet-
connected devices that either lack a browser to perform a user-agent-
based authorization or are input constrained to the extent that
requiring the user to input text in order to authenticate during the
authorization flow is impractical. It enables OAuth clients on such
devices (like smart TVs, media consoles, digital picture frames, and
printers) to obtain user authorization to access protected resources
by using a user agent on a separate device.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8628.

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described in the Simplified BSD License.

1. Introduction

This OAuth 2.0 [RFC6749] protocol extension enables OAuth clients to
request user authorization from applications on devices that have
limited input capabilities or lack a suitable browser. Such devices
include smart TVs, media consoles, picture frames, and printers,
which lack an easy input method or a suitable browser required for
traditional OAuth interactions. The authorization flow defined by
this specification, sometimes referred to as the "device flow",
instructs the user to review the authorization request on a secondary
device, such as a smartphone, which does have the requisite input and
browser capabilities to complete the user interaction.

The device authorization grant is not intended to replace browser-
based OAuth in native apps on capable devices like smartphones.
Those apps should follow the practices specified in "OAuth 2.0 for
Native Apps" [RFC8252].

The operating requirements for using this authorization grant type
are:

(1) The device is already connected to the Internet.

(2) The device is able to make outbound HTTPS requests.

(3) The device is able to display or otherwise communicate a URI and

code sequence to the user.

(4) The user has a secondary device (e.g., personal computer or
smartphone) from which they can process the request.

As the device authorization grant does not require two-way
communication between the OAuth client on the device and the user
agent (unlike other OAuth 2 grant types, such as the authorization
code and implicit grant types), it supports several use cases that
cannot be served by those other approaches.

Instead of interacting directly with the end user's user agent (i.e.,
browser), the device client instructs the end user to use another
computer or device and connect to the authorization server to approve
the access request. Since the protocol supports clients that can't
receive incoming requests, clients poll the authorization server
repeatedly until the end user completes the approval process.

The device client typically chooses the set of authorization servers
to support (i.e., its own authorization server or those of providers
with which it has relationships). It is common for the device client
to support only one authorization server, such as in the case of a TV
application for a specific media provider that supports only that
media provider's authorization server. The user may not yet have an
established relationship with that authorization provider, though one
can potentially be set up during the authorization flow.

Figure 1: Device Authorization Flow

The device authorization flow illustrated in Figure 1 includes the
following steps:

(A) The client requests access from the authorization server and

includes its client identifier in the request.

(B) The authorization server issues a device code and an end-user

code and provides the end-user verification URI.

(C) The client instructs the end user to use a user agent on another

device and visit the provided end-user verification URI. The
client provides the user with the end-user code to enter in
order to review the authorization request.

(D) The authorization server authenticates the end user (via the

user agent), and prompts the user to input the user code
provided by the device client. The authorization server
validates the user code provided by the user, and prompts the
user to accept or decline the request.

(E) While the end user reviews the client's request (step D), the

client repeatedly polls the authorization server to find out if

the user completed the user authorization step. The client

includes the device code and its client identifier.

(F) The authorization server validates the device code provided by

the client and responds with the access token if the client is

granted access, an error if they are denied access, or an

indication that the client should continue to poll.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Protocol

3.1. Device Authorization Request

This specification defines a new OAuth endpoint: the device
authorization endpoint. This is separate from the OAuth
authorization endpoint defined in [RFC6749] with which the user
interacts via a user agent (i.e., a browser). By comparison, when
using the device authorization endpoint, the OAuth client on the
device interacts with the authorization server directly without
presenting the request in a user agent, and the end user authorizes
the request on a separate device. This interaction is defined as
follows.

The client initiates the authorization flow by requesting a set of
verification codes from the authorization server by making an HTTP
"POST" request to the device authorization endpoint.

The client makes a device authorization request to the device
authorization endpoint by including the following parameters using
the "application/x-www-form-urlencoded" format, per Appendix B of
[RFC6749], with a character encoding of UTF-8 in the HTTP request
entity-body:

client_id

REQUIRED if the client is not authenticating with the
authorization server as described in Section 3.2.1. of [RFC6749].
The client identifier as described in Section 2.2 of [RFC6749].

scope

OPTIONAL. The scope of the access request as defined by
Section 3.3 of [RFC6749].

All requests from the device MUST use the Transport Layer Security
(TLS) protocol [RFC8446] and implement the best practices of BCP 195
[RFC7525].

Parameters sent without a value MUST be treated as if they were
omitted from the request. The authorization server MUST ignore
unrecognized request parameters. Request and response parameters
MUST NOT be included more than once.

The client authentication requirements of Section 3.2.1 of [RFC6749]
apply to requests on this endpoint, which means that confidential
clients (those that have established client credentials) authenticate
in the same manner as when making requests to the token endpoint, and
public clients provide the "client_id" parameter to identify
themselves.

Due to the polling nature of this protocol (as specified in
Section 3.4), care is needed to avoid overloading the capacity of the
token endpoint. To avoid unneeded requests on the token endpoint,
the client SHOULD only commence a device authorization request when
prompted by the user and not automatically, such as when the app
starts or when the previous authorization session expires or fails.

3.2. Device Authorization Response

In response, the authorization server generates a unique device
verification code and an end-user code that are valid for a limited
time and includes them in the HTTP response body using the
"application/json" format [RFC8259] with a 200 (OK) status code. The
response contains the following parameters:

device_code

REQUIRED. The device verification code.

user_code

REQUIRED. The end-user verification code.

verification_uri

REQUIRED. The end-user verification URI on the authorization
server. The URI should be short and easy to remember as end users
will be asked to manually type it into their user agent.

verification_uri_complete

OPTIONAL. A verification URI that includes the "user_code" (or
other information with the same function as the "user_code"),
which is designed for non-textual transmission.

expires_in

REQUIRED. The lifetime in seconds of the "device_code" and
"user_code".

interval

OPTIONAL. The minimum amount of time in seconds that the client
SHOULD wait between polling requests to the token endpoint. If no
value is provided, clients MUST use 5 as the default.

In the event of an error (such as an invalidly configured client),
the authorization server responds in the same way as the token
endpoint specified in Section 5.2 of [RFC6749].

3.3. User Interaction

After receiving a successful authorization response, the client
displays or otherwise communicates the "user_code" and the
"verification_uri" to the end user and instructs them to visit the
URI in a user agent on a secondary device (for example, in a browser
on their mobile phone) and enter the user code.

Figure 2: Example User Instruction

The authorizing user navigates to the "verification_uri" and
authenticates with the authorization server in a secure TLS-protected
[RFC8446] session. The authorization server prompts the end user to
identify the device authorization session by entering the "user_code"
provided by the client. The authorization server should then inform
the user about the action they are undertaking and ask them to
approve or deny the request. Once the user interaction is complete,
the server instructs the user to return to their device.

During the user interaction, the device continuously polls the token
endpoint with the "device_code", as detailed in Section 3.4, until
the user completes the interaction, the code expires, or another
error occurs. The "device_code" is not intended for the end user
directly; thus, it should not be displayed during the interaction to
avoid confusing the end user.

Authorization servers supporting this specification MUST implement a
user-interaction sequence that starts with the user navigating to
"verification_uri" and continues with them supplying the "user_code"
at some stage during the interaction. Other than that, the exact
sequence and implementation of the user interaction is up to the
authorization server; for example, the authorization server may
enable new users to sign up for an account during the authorization
flow or add additional security verification steps.

It is NOT RECOMMENDED for authorization servers to include the user
code ("user_code") in the verification URI ("verification_uri"), as
this increases the length and complexity of the URI that the user
must type. While the user must still type a similar number of
characters with the "user_code" separated, once they successfully
navigate to the "verification_uri", any errors in entering the code
can be highlighted by the authorization server to improve the user
experience. The next section documents the user interaction with
"verification_uri_complete", which is designed to carry both pieces
of information.

3.3.1. Non-Textual Verification URI Optimization

When "verification_uri_complete" is included in the authorization
response (Section 3.2), clients MAY present this URI in a non-textual
manner using any method that results in the browser being opened with
the URI, such as with QR (Quick Response) codes or NFC (Near Field
Communication), to save the user from typing the URI.

For usability reasons, it is RECOMMENDED for clients to still display
the textual verification URI ("verification_uri") for users who are
not able to use such a shortcut. Clients MUST still display the
"user_code", as the authorization server will require the user to
confirm it to disambiguate devices or as remote phishing mitigation
(see Section 5.4).

If the user starts the user interaction by navigating to
"verification_uri_complete", then the user interaction described in
Section 3.3 is still followed, with the optimization that the user
does not need to type in the "user_code". The server SHOULD display
the "user_code" to the user and ask them to verify that it matches
the "user_code" being displayed on the device to confirm they are
authorizing the correct device. As before, in addition to taking
steps to confirm the identity of the device, the user should also be
afforded the choice to approve or deny the authorization request.

of the Complete Verification URI

3.4. Device Access Token Request

After displaying instructions to the user, the client creates an
access token request and sends it to the token endpoint (as defined
by Section 3.2 of [RFC6749]) with a "grant_type" of
"urn:ietf:params:oauth:grant-type:device_code". This is an extension
grant type (as defined by Section 4.5 of [RFC6749]) created by this
specification, with the following parameters:

grant_type

REQUIRED. Value MUST be set to
"urn:ietf:params:oauth:grant-type:device_code".

If the client was issued client credentials (or assigned other
authentication requirements), the client MUST authenticate with the
authorization server as described in Section 3.2.1 of [RFC6749].
Note that there are security implications of statically distributed
client credentials; see Section 5.6.

The response to this request is defined in Section 3.5. Unlike other
OAuth grant types, it is expected for the client to try the access
token request repeatedly in a polling fashion based on the error code
in the response.

3.5. Device Access Token Response

If the user has approved the grant, the token endpoint responds with
a success response defined in Section 5.1 of [RFC6749]; otherwise, it
responds with an error, as defined in Section 5.2 of [RFC6749].

In addition to the error codes defined in Section 5.2 of [RFC6749],
the following error codes are specified for use with the device
authorization grant in token endpoint responses:

authorization_pending

The authorization request is still pending as the end user hasn't
yet completed the user-interaction steps (Section 3.3). The
client SHOULD repeat the access token request to the token
endpoint (a process known as polling). Before each new request,
the client MUST wait at least the number of seconds specified by
the "interval" parameter of the device authorization response (see
Section 3.2), or 5 seconds if none was provided, and respect any
increase in the polling interval required by the "slow_down"
error.

slow_down

A variant of "authorization_pending", the authorization request is
still pending and polling should continue, but the interval MUST
be increased by 5 seconds for this and all subsequent requests.

access_denied

The authorization request was denied.

expired_token

The "device_code" has expired, and the device authorization
session has concluded. The client MAY commence a new device
authorization request but SHOULD wait for user interaction before
restarting to avoid unnecessary polling.

The "authorization_pending" and "slow_down" error codes define
particularly unique behavior, as they indicate that the OAuth client
should continue to poll the token endpoint by repeating the token
request (implementing the precise behavior defined above). If the
client receives an error response with any other error code, it MUST
stop polling and SHOULD react accordingly, for example, by displaying
an error to the user.

On encountering a connection timeout, clients MUST unilaterally
reduce their polling frequency before retrying. The use of an
exponential backoff algorithm to achieve this, such as doubling the
polling interval on each such connection timeout, is RECOMMENDED.

The assumption of this specification is that the separate device on
which the user is authorizing the request does not have a way to
communicate back to the device with the OAuth client. This protocol
only requires a one-way channel in order to maximize the viability of
the protocol in restricted environments, like an application running
on a TV that is only capable of outbound requests. If a return
channel were to exist for the chosen user-interaction interface, then
the device MAY wait until notified on that channel that the user has
completed the action before initiating the token request (as an
alternative to polling). Such behavior is, however, outside the
scope of this specification.

4. Discovery Metadata

Support for this protocol is declared in OAuth 2.0 Authorization
Server Metadata [RFC8414] as follows. The value
"urn:ietf:params:oauth:grant-type:device_code" is included in values
of the "grant_types_supported" key, and the following new key value
pair is added:

5. Security Considerations

5.1. User Code Brute Forcing

Since the user code is typed by the user, shorter codes are more
desirable for usability reasons. This means the entropy is typically
less than would be used for the device code or other OAuth bearer
token types where the code length does not impact usability.
Therefore, it is recommended that the server rate-limit user code
attempts.

The user code SHOULD have enough entropy that, when combined with
rate-limiting and other mitigations, a brute-force attack becomes
infeasible. For example, it's generally held that 128-bit symmetric
keys for encryption are seen as good enough today because an attacker
has to put in 2^96 work to have a 2^-32 chance of guessing correctly
via brute force. The rate-limiting and finite lifetime on the user
code place an artificial limit on the amount of work an attacker can
"do". If, for instance, one uses an 8-character base 20 user code
(with roughly 34.5 bits of entropy), the rate-limiting interval and
validity period would need to only allow 5 attempts in order to get
the same 2^-32 probability of success by random guessing.

A successful brute forcing of the user code would enable the attacker
to approve the authorization grant with their own credentials, after
which the device would receive a device authorization grant linked to
the attacker's account. This is the opposite scenario to an OAuth
bearer token being brute forced, whereby the attacker gains control
of the victim's authorization grant. Such attacks may not always
make economic sense. For example, for a video app, the device owner
may then be able to purchase movies using the attacker's account
(though even in this case a privacy risk would still remain and thus
is important to protect against). Furthermore, some uses of the
device flow give the granting account the ability to perform actions
that need to be protected, such as controlling the device.

The precise length of the user code and the entropy contained within
is at the discretion of the authorization server, which needs to
consider the sensitivity of their specific protected resources, the
practicality of the code length from a usability standpoint, and any
mitigations that are in place, such as rate-limiting, when
determining the user code format.

5.2. Device Code Brute Forcing

An attacker who guesses the device code would be able to potentially
obtain the authorization code once the user completes the flow. As
the device code is not displayed to the user and thus there are no
usability considerations on the length, a very high entropy code
SHOULD be used.

5.3. Device Trustworthiness

Unlike other native application OAuth 2.0 flows, the device
requesting the authorization is not the same as the device from which
the user grants access. Thus, signals from the approving user's
session and device are not always relevant to the trustworthiness of
the client device.

Note that if an authorization server used with this flow is
malicious, then it could perform a man-in-the-middle attack on the
backchannel flow to another authorization server. In this scenario,
the man-in-the-middle is not completely hidden from sight, as the end
user would end up on the authorization page of the wrong service,
giving them an opportunity to notice that the URL in the browser's
address bar is wrong. For this to be possible, the device
manufacturer must either be the attacker and shipping a device
intended to perform the man-in-the-middle attack, or be using an
authorization server that is controlled by an attacker, possibly
because the attacker compromised the authorization server used by the
device. In part, the person purchasing the device is counting on the
manufacturer and its business partners to be trustworthy.

5.4. Remote Phishing

It is possible for the device flow to be initiated on a device in an
attacker's possession. For example, an attacker might send an email
instructing the target user to visit the verification URL and enter
the user code. To mitigate such an attack, it is RECOMMENDED to
inform the user that they are authorizing a device during the user-
interaction step (see Section 3.3) and to confirm that the device is
in their possession. The authorization server SHOULD display
information about the device so that the user could notice if a
software client was attempting to impersonate a hardware device.

For authorization servers that support the
"verification_uri_complete" optimization discussed in Section 3.3.1,
it is particularly important to confirm that the device is in the
user's possession, as the user no longer has to type in the code
being displayed on the device manually. One suggestion is to display
the code during the authorization flow and ask the user to verify
that the same code is currently being displayed on the device they
are setting up.

The user code needs to have a long enough lifetime to be useable
(allowing the user to retrieve their secondary device, navigate to
the verification URI, log in, etc.) but should be sufficiently short
to limit the usability of a code obtained for phishing. This doesn't
prevent a phisher from presenting a fresh token, particularly if they
are interacting with the user in real time, but it does limit the
viability of codes sent over email or text message.

5.5. Session Spying

While the device is pending authorization, it may be possible for a
malicious user to physically spy on the device user interface (by
viewing the screen on which it's displayed, for example) and hijack
the session by completing the authorization faster than the user that
initiated it. Devices SHOULD take into account the operating
environment when considering how to communicate the code to the user
to reduce the chances it will be observed by a malicious user.

5.6. Non-Confidential Clients

Device clients are generally incapable of maintaining the
confidentiality of their credentials, as users in possession of the
device can reverse-engineer it and extract the credentials.
Therefore, unless additional measures are taken, they should be
treated as public clients (as defined by Section 2.1 of [RFC6749]),
which are susceptible to impersonation. The security considerations
of Section 5.3.1 of [RFC6819] and Sections 8.5 and 8.6 of [RFC8252]
apply to such clients.

The user may also be able to obtain the "device_code" and/or other
OAuth bearer tokens issued to their client, which would allow them to
use their own authorization grant directly by impersonating the
client. Given that the user in possession of the client credentials
can already impersonate the client and create a new authorization
grant (with a new "device_code"), this doesn't represent a separate
impersonation vector.

5.7. Non-Visual Code Transmission

There is no requirement that the user code be displayed by the device
visually. Other methods of one-way communication can potentially be
used, such as text-to-speech audio or Bluetooth Low Energy. To
mitigate an attack in which a malicious user can bootstrap their
credentials on a device not in their control, it is RECOMMENDED that
any chosen communication channel only be accessible by people in
close proximity, for example, users who can see or hear the device.

6. Usability Considerations

This section is a non-normative discussion of usability
considerations.

6.1. User Code Recommendations

For many users, their nearest Internet-connected device will be their
mobile phone; typically, these devices offer input methods that are
more time-consuming than a computer keyboard to change the case or
input numbers. To improve usability (improving entry speed and
reducing retries), the limitations of such devices should be taken
into account when selecting the user code character set.

One way to improve input speed is to restrict the character set to
case-insensitive A-Z characters, with no digits. These characters
can typically be entered on a mobile keyboard without using modifier
keys. Further removing vowels to avoid randomly creating words
results in the base 20 character set "BCDFGHJKLMNPQRSTVWXZ". Dashes
or other punctuation may be included for readability.

An example user code following this guideline, "WDJB-MJHT", contains
8 significant characters and has dashes added for end-user
readability. The resulting entropy is 20^8.

Pure numeric codes are also a good choice for usability, especially
for clients targeting locales where A-Z character keyboards are not
used, though the length of such a code needs to be longer to maintain
high entropy.

An example numeric user code that contains 9 significant digits and
dashes added for end-user readability with an entropy of 10^9 is
"019-450-730".

When processing the inputted user code, the server should strip
dashes and other punctuation that it added for readability (making
the inclusion of such punctuation by the user optional). For codes
using only characters in the A-Z range, as with the base 20 charset
defined above, the user's input should be uppercased before a
comparison to account for the fact that the user may input the
equivalent lowercase characters. Further stripping of all characters
outside the chosen character set is recommended to reduce instances
where an errantly typed character (like a space character)
invalidates otherwise valid input.

It is RECOMMENDED to avoid character sets that contain two or more
characters that can easily be confused with each other, like "0" and
"O" or "1", "l" and "I". Furthermore, to the extent practical, when
a character set contains a character that may be confused with
characters outside the character set, a character outside the set MAY
be substituted with the one in the character set with which it is
commonly confused; for example, "O" may be substituted for "0" when
using the numerical 0-9 character set.

6.2. Non-Browser User Interaction

Devices and authorization servers MAY negotiate an alternative code
transmission and user-interaction method in addition to the one
described in Section 3.3. Such an alternative user-interaction flow
could obviate the need for a browser and manual input of the code,
for example, by using Bluetooth to transmit the code to the
authorization server's companion app. Such interaction methods can
utilize this protocol as, ultimately, the user just needs to identify
the authorization session to the authorization server; however, user
interaction other than through the verification URI is outside the
scope of this specification.

7. IANA Considerations

7.1. OAuth Parameter Registration

This specification registers the following values in the IANA "OAuth
Parameters" registry [IANA.OAuth.Parameters] established by
[RFC6749].

Acknowledgements

The starting point for this document was the Internet-Draft
draft-recordon-oauth-v2-device, authored by David Recordon and Brent
Goldman, which itself was based on content in draft versions of the
OAuth 2.0 protocol specification removed prior to publication due to
a then-lack of sufficient deployment expertise. Thank you to the
OAuth Working Group members who contributed to those earlier drafts.

This document was produced in the OAuth Working Group under the
chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig, with
Benjamin Kaduk, Kathleen Moriarty, and Eric Rescorla serving as
Security Area Directors.

The following individuals contributed ideas, feedback, and wording
that shaped and formed the final specification: